Perhaps the best endorsement comes from the LA Times, who list several points in making their decision. But in the end, the best point is a metaphorical statement about the candidates that I find quite true in my own feelings about them.
In the language of metaphor, Clinton is an essay, solid and reasoned; Obama is a poem, lyric and filled with possibility. Clinton would be a valuable and competent executive, but Obama matches her in substance and adds something that the nation has been missing far too long -- a sense of aspiration.
This rings true for me, and I fully admit that my support for Obama comes more from my sense of him as a person, and less about the distinctions between him and Clinton. Hillary would no doubt be a better president than either of the two GOP front-runners (McCain and Romney), but Obama inspires me, and that has been what I have been seeking in a presidential candidate for all of my adult life.